The microstate geometries 
[1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]} are smooth horizonless solutions with the same asymptotic structure as a black hole or a black ring.
So far, these solutions have been thought of as one of ways to resolve the problem of black hole information loss.
The idea to describe black hole microstates by horizonless geometries first originated from the works on fuzzballs of Mathur [15]}, [16]}, [17]}.
The existence itself of such solutions should be surprising because in the earlier works [18]}, [19]}, [20]}, [21]}, it is a well-known fact that smooth soliton solutions in four dimensions are completely excluded.
However, in five dimensions, at least, in five dimensional supergravity, the no-go theorem does not hold because the spacetime admits the spatial cross sections with non-trivial second homology and the Chern-Simons interactions.
